Deep Blue and Highly Emissive ZnS-Passivated InP QDs: Facile Synthesis, Characterization, and Deciphering of Their Ultrafast-to-Slow Photodynamics

InP-based quantum dots (QDs) are an environment-friendly alternative to their heavy metal-ion-based counterparts. Herein we report a simple procedure for synthesizing blue emissive InP QDs using oleic acid and oleylamine as surface ligands, yielding ultrasmall QDs with average sizes of 1.74 and 1.81 nm, respectively. Consecutive thin coating with ZnS increased the size of these QDs to 4.11 and 4.15 nm, respectively, alongside a significant enhancement of their emission intensities centered at ∼410 nm and ∼430 nm, respectively. Pure phase synthesis of these deep-blue emissive QDs is confirmed by powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). Armed with femtosecond to millisecond time-resolved spectroscopic techniques, we decipher the energy pathways, reflecting the effect of successive ZnS passivation on the charge carrier (electrons and holes) dynamics in the deep-blue emissive InP, InP/ZnS, and InP/ZnS/ZnS QDs. Successive coating of the InP QDs increases the intraband relaxation times from 200 to 700 fs and the lifetime of the hot electrons from 2 to 8 ps. The lifetime of the cold holes also increase from 1 to 4 ps, and remarkably, the Auger recombination escalates from 15 to 165 ps. The coating also drastically decreases the quenching by the molecular oxygen of the trapped charge carriers at the surfaces of the QDs. Our results provide clues to push further the emission of InP QDs into more energetically spectral regions and to increase the fluorescence quantum yield, targeting the construction of efficient UV-emissive light-emitting devices (LEDs).

InP core sizes and emission wavelength maxima reported previously and in the present work S-17 16. Table S2 Time constants, preexponential factors and contributions to the signal obtained from emission decays of coated and uncoated InP QDs with surface ligand oleic acid S-18 17. Table S3 Time constants, preexponential factors and contributions to the signal obtained from emission decays of coated and uncoated InP QDs with surface ligand oleyl amine S-19 18.

Synthesis of blue emitting OAC-and OAM-capped InP QDs
In a typical synthesis procedure, 29.2 mg (5 mM) In(OAc) 3 and 1.8 mg (0.25 mM) Zn(OAc) 3 were mixed in 15 mL of ODE and 0.1 M of OAC (Scheme S1). The mixture was stirred and degassed at 130 °C for half an hour before purging N 2 gas to dissolve completely In(OAc) 3 and Zn(OAc) 3 . After ensuring that the system is homogeneous and in inert atmosphere, the temperature of the solution is increased to 170 °C. Then 0.25 mL of the phosphorous precursor TDMAP was added, and the reaction was then held for 30 minutes to ensure the successful S-3 synthesis of InP QDs. Addition of very small amount of Zn-acetate passivate the surface dangling bonds by forming a In(Zn)P outer layer that protects the InP core from oxidation even after the washing and redispersion process. 1,2,3,4 The formation of this In(Zn)P layer also reduces the probability of secondary nucleation of the InP cores and obstruct Ostawld ripening. 2,4 The InP QDs were then precipitated with ethanol and redispersed in hexane for further studies.
We followed the same procedure for the synthesis of oleylamine (OAM) capped InP QDs but replacing the OAC with 0.1 M of OAM.

Synthesis of blue emitting OAC-and OAM-capped InP/ZnS QDs
The synthesized InP QDs were used to produce OAC capped InP/ZnS QDs. To do so, we mixed a 10 mL solution consisting of Zn(OAc) 2 in a mixture of OAC in ODE along with 10 mL of OAC capped InP QDs. The solution was heated at 200 °C for 30 minutes under N 2 environment before adding 1-dodecanthiol (1-DDT). After adding 1-DDT, the temperature of the mixture was quickly increased to 280 °C and kept for 1 hour to coat the first ZnS shell over the InP QDs. The synthesized InP/ZnS QDs were then precipitated in ethanol and redispersed in hexane for further studies.
For the synthesis of OAM-capped InP/ZnS QDs, we have followed the same procedure described above. The only difference is that here we have used a 10 mL solution consisting of Zn(OAc) 2 in a mixture of OAM in ODE along with 10 mL of OAM capped InP QDs before adding 1-DDT.

Synthesis of blue emitting OAC-and OAM-capped InP/ZnS/ZnS QDs
The InP/ZnS QDs synthesized in the first step were then employed to produce OAC capped InP/ZnS/ZnS QDs. We mixed a 5 mL solution consisting of Zn(OAc) 2  Blue emitting OAM-capped InP/ZnS/ZnS QDs were synthesized using the same procedure. The only difference is the replacement of OAC by OAM.

Transmission Electron Microscopy (TEM)
High-resolution TEM (HRTEM) images of the samples were recorded using a JEOL 2100

Steady-state absorption, emission and picosecond (ps) time resolved observation
A JASCO V-670 spectrophotometer was used to record steady-state UV-visible absorption spectra using 10-mm path length quartz cuvettes. The excitation and emission spectra were recorded on a Fluoromax-4 (Jobin-Yvone) spectrofluorometer using a quartz cuvette of 10-mm path length. Excited-state lifetimes were measured using a ps time-correlated single-photon counting system (FluoTime 200, PicoQuant) described previously. 5 The samples were excited by a 0.8 mW, 40 ps pulsed (20 MHz) LDH 400 laser centered at 371 nm. The fluorescence signal was collected at magic angle and at different observation wavelengths. The instrument response function (IRF) was 70 ps. The decays were deconvoluted and fitted to a multiexponential function using the FLUOFIT package (PicoQuant), which allows single and global fits. The quality of the fits as well as the number of exponentials were checked based on the reduced χ 2 values (which were always below <1.2) and the distributions of the residuals.

Fluorescence Upconversion Spectroscopy
The femtosecond (fs) time-resolved emission transients were collected using a fluorescence upconversion setup (FOG100, CDP Systems) described elsewhere. 6,7 The samples were excited at 360 nm by a fs-pulse from the second harmonic of Ti:Sapphire oscillator (MaiTai Spectra Physics) output (720 nm). The samples were placed in a 1-mm rotating cell to avoid reexcitation and photodegradation, The IRF of the setup is ∼180 fs measured as the Raman signal of the solvent. The recorded transients at selected gated wavelengths were analyzed by convoluting a multiexponential function with the IRF to fit the experimental data. The estimated errors for the calculated time constants were below 15% in all cases.

Femtosecond Transient Absorption Spectroscopy
The used fs transient UV−vis−NIR absorption setup has been described elsewhere. 8  Conversion). A small portion of the rest of the fundamental beam (~200 μW) was directed to Sapphire crystal for white light continuum generation. The used pump intensity was ∼500 μW (spot size at the sample was 280 μm) and the excitation wavelength was 340 nm. The instrumental response function (IRF) was 160 fs. All spectra analyzed in the UV−vis region were corrected for the chirp of the white light continuum. Transient absorption measurements were performed in the spectral ranges of 430−620 (UV−vis region) and 870 -1050 nm (NIR).
To avoid sample degradation, the samples were placed in a 1-mm rotating cell. The data were analyzed using a multiexponential global fit program. The quality of the global fit was checked by examining the fits at different wavelengths and χ 2 .

Flash Photolysis Measurements
The nanosecond to second flash photolysis setup was described elsewhere. 9

SN1:
The average radiative ( ) and non-radiative ( ) rate constants were calculated using the QY and the averaged lifetime, , using the following two equations: 10,11 (Eq. 1)

SN2:
The average number of photons absorbed per quantum dot per pulse is determined by the following procedure. 12,13 The data presented The energy per photon, E phot , at a wavelength of 400 nm is given by the Planck-Einstein relation, The photon fluence (per pulse per area) is thus given by the ratio between the pump fluence and the photon energy, according to,       nm. The solid lines are from the best multiexponential fits. S-17